Recently, personal digital assistants acronymed as PDAs and cellular phones have undergone explosive growth in demand. Some such systems have digital camera or digital video functions added to them. To implement these functions, CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) sensors are now used as image pickup devices. To reduce the sizes of such systems, it is preferable to use an image pickup device having a relatively small light receiving area. In this case, a sensible tradeoff between size reductions and cost savings must be made while the performance of an optical system is kept high. Size reductions are now achieved by reducing the number of lenses used. On the other hand, cost reductions by use of a fewer step, for instance, is now achieved by use of a fabrication process wherein lenses are formed under pressure in a lens holder.
For reductions in the number of lenses that form an optical system, it is necessary to use aspheric lenses. For the fabrication of such aspheric lenses, use is generally made of a fabrication process wherein a preform is pressed in a state softened by heating (hereinafter called the prior art lens processing). With this prior art lens processing, an aspheric lens is formed larger than the required outer diameter, and rounding is carried out in such a way as to incorporate it in a lens barrel. For this reason, for instance, the thickness of the outer periphery of the lens at the necessary outer diameter will become larger than that of the lens during rounding. A reduction in the number of lenses for compactness will result in an increase in lens thickness, because the refracting power of each of lenses inclusive of a positive lens will become strong. For this reason and to give the lens a sufficient peripheral thickness, the peripheral thickness of the lens at the necessary outer diameter will become far larger. Thus, no sufficient effect on size reductions will still be obtained.
On the other hand, Patent Publication 1 says nothing about not only size reductions but also conditions for size reductions.
Patent Publication 1
JP (A) 61-114822